gh-96143: Improve perf profiler docs

Author: erlend-aasland

Doc/howto/perf_profiling.rst

@@ -25,7 +25,7 @@ fly before the execution of every Python function and it will teach ``perf`` the
 relationship between this piece of code and the associated Python function using
 `perf map files`_.
 
-.. warning::
+.. note::
 
     Support for the ``perf`` profiler is only currently available for Linux on
     selected architectures. Check the output of the configure build step or
@@ -51,11 +51,11 @@ For example, consider the following script:
     if __name__ == "__main__":
         baz(1000000)
 
-We can run perf to sample CPU stack traces at 9999 Hertz:
+We can run ``perf`` to sample CPU stack traces at 9999 Hertz::
 
     $ perf record -F 9999 -g -o perf.data python my_script.py
 
-Then we can use perf report to analyze the data:
+Then we can use ``perf`` report to analyze the data:
 
 .. code-block:: shell-session
 
@@ -101,7 +101,7 @@ As you can see here, the Python functions are not shown in the output, only ``_P
 functions use the same C function to evaluate bytecode so we cannot know which Python function corresponds to which
 bytecode-evaluating function.
 
-Instead, if we run the same experiment with perf support activated we get:
+Instead, if we run the same experiment with ``perf`` support activated we get:
 
 .. code-block:: shell-session
 
@@ -147,19 +147,27 @@ Instead, if we run the same experiment with perf support activated we get:
 
 
 
-Enabling perf profiling mode
-----------------------------
+How to enable the ``perf`` profiling mode
+-----------------------------------------
 
-There are two main ways to activate the perf profiling mode. If you want it to be
-active since the start of the Python interpreter, you can use the `-Xperf` option:
+There are three ways to activate the ``perf`` profiling mode:
+using the :option:`-Xperf <-X>` command-line option,
+using the :envvar:`PYTHONPERFSUPPORT` environment variable,
+and using the :func:`sys.activate_stack_trampoline` and
+:func:`sys.deactivate_stack_trampoline` APIs.
+If you want profiling to be active when you start the Python interpreter,
+use the :option:`-Xperf <-X>` option::
 
     $ python -Xperf my_script.py
 
-You can also set the :envvar:`PYTHONPERFSUPPORT` to a nonzero value to actiavate perf
-profiling mode globally.
+If you need to activate the ``perf`` profiling mode globally,
+set the environment variable :envvar:`PYTHONPERFSUPPORT`
+to a nonzero value.
 
-There is also support for dynamically activating and deactivating the perf
-profiling mode by using the APIs in the :mod:`sys` module:
+If you need to dynamically activate and deactivate the ``perf`` profiling mode
+in response to a signal or other communication mechanisms with your process,
+use the :func:`sys.activate_stack_trampoline` and
+:func:`sys.deactivate_stack_trampoline` APIs:
 
 .. code-block:: python
 
@@ -172,27 +180,22 @@ profiling mode by using the APIs in the :mod:`sys` module:
 
     # Perf profiling is not active anymore
 
-These APIs can be handy if you want to activate/deactivate profiling mode in
-response to a signal or other communication mechanism with your process.
-
-
-
-Now we can analyze the data with ``perf report``:
+Now we can analyze the data with ``perf report``::
 
     $ perf report -g -i perf.data
 
 
 How to obtain the best results
--------------------------------
+------------------------------
 
 For the best results, Python should be compiled with
 ``CFLAGS="-fno-omit-frame-pointer -mno-omit-leaf-frame-pointer"`` as this allows
 profilers to unwind using only the frame pointer and not on DWARF debug
-information. This is because as the code that is interposed to allow perf
+information. This is because as the code that is interposed to allow ``perf``
 support is dynamically generated it doesn't have any DWARF debugging information
 available.
 
-You can check if you system has been compiled with this flag by running:
+You can check if you system has been compiled with this flag by running::
 
     $ python -m sysconfig | grep 'no-omit-frame-pointer'
 

Doc/library/sys.rst

@@ -1524,6 +1524,35 @@ always available.
       This function has been added on a provisional basis (see :pep:`411`
       for details.)  Use it only for debugging purposes.
 
+.. function:: activate_stack_trampoline(backend, /)
+
+   Activate the stack profiler trampoline *backend*.
+   The only supported backend is `"perf"`.
+
+   .. availability:: Linux.
+
+   .. versionadded:: 3.12
+
+   .. seealso::
+
+      :ref:`perf_profiling`
+
+.. function:: deactivate_stack_trampoline()
+
+   Deactivate the stack profiler trampoline.
+
+   .. availability:: Linux.
+
+   .. versionadded:: 3.12
+
+.. function:: is_stack_trampoline_active()
+
+   Return ``True`` if the stack profiler trampoline is active.
+
+   .. availability:: Linux.
+
+   .. versionadded:: 3.12
+
 .. function:: _enablelegacywindowsfsencoding()
 
    Changes the :term:`filesystem encoding and error handler` to 'mbcs' and

Doc/using/cmdline.rst

@@ -535,12 +535,13 @@ Miscellaneous options
      development (running from the source tree) then the default is "off".
      Note that the "importlib_bootstrap" and "importlib_bootstrap_external"
      frozen modules are always used, even if this flag is set to "off".
-   * ``-X perf`` to activate compatibility mode with the ``perf`` profiler.
+   * ``-X perf`` activates compatibility mode with the ``perf`` profiler.
      When this option is activated, the Linux ``perf`` profiler will be able to
      report Python calls. This option is only available on some platforms and
      will do nothing if is not supported on the current system. The default value
-     is "off". See also :envvar:`PYTHONPERFSUPPORT` and :ref:`perf_profiling`
-     for more information.
+     is "off". See :ref:`perf_profiling` for more details.
+     This option is equivalent to setting the environment variable
+     :envvar:`PYTHONPERFSUPPORT` to ``1``.
 
    It also allows passing arbitrary values and retrieving them through the
    :data:`sys._xoptions` dictionary.
@@ -1036,8 +1037,13 @@ conflict.
 .. envvar:: PYTHONPERFSUPPORT
 
    If this variable is set to a nonzero value, it activates compatibility mode
-   with the ``perf`` profiler so Python calls can be detected by it. See the
-   :ref:`perf_profiling` section for more information.
+   with the ``perf`` profiler so Python calls can be detected by it.
+   This is exactly equivalent to setting :option:`-X perf <-X>`
+   on the command line.
+
+   If set to ``0``, disable ``perf`` profiler support.
+
+   See :ref:`perf_profiling` for more details.
 
    .. versionadded:: 3.12
 

Doc/whatsnew/3.12.rst

@@ -74,6 +74,14 @@ Important deprecations, removals or restrictions:
 New Features
 ============
 
+* Add :ref:`perf_profiling` through the new command-line option
+  :option:`-X perf <-X>`, as well as the new
+  :func:`sys.activate_stack_trampoline`,
+  :func:`sys.deactivate_stack_trampoline`,
+  and :func:`sys.is_stack_trampoline_active` APIs.
+  (Contributed by Pablo Galindo and Christian Heimes
+  with contributions from Gregory P. Smith [Google] and Mark Shannon
+  in :gh:`96123`.)
 
 
 Other Language Changes
@@ -126,6 +134,19 @@ sqlite3
   (Contributed by Erlend E. Aasland in :gh:`77617`.)
 
 
+sys
+---
+
+* Add :func:`sys.activate_stack_trampoline` and
+  :func:`sys.deactivate_stack_trampoline` for activating and deactivating
+  stack profiler trampolines,
+  and :func:`sys.is_stack_trampoline_active` for querying if stack profiler
+  trampolines are active.
+  (Contributed by Pablo Galindo and Christian Heimes
+  with contributions from Gregory P. Smith [Google] and Mark Shannon
+  in :gh:`96123`.)
+
+
 Optimizations
 =============
 

Misc/NEWS.d/next/Core and Builtins/2022-08-20-18-36-40.gh-issue-96143.nh3GFM.rst

@@ -1,7 +1,6 @@
-Add a new ``-X perf`` Python command line option as well as
-:func:`sys.activate_stack_trampoline` and :func:`sys.deactivate_stack_trampoline`
-function in the :mod:`sys` module that allows to set/unset the interpreter in a
-way that the Linux ``perf`` profiler can detect Python calls. The new
-:func:`sys.is_stack_trampoline_active` function allows to query the state of the
-perf trampoline. Design by Pablo Galindo. Patch by Pablo Galindo and Christian Heimes
+Add :ref:`perf_profiling` through the new command-line option
+:option:`-X perf <-X>`, as well as the new
+:func:`sys.activate_stack_trampoline`, :func:`sys.deactivate_stack_trampoline`,
+and :func:`sys.is_stack_trampoline_active` APIs.
+Design by Pablo Galindo. Patch by Pablo Galindo and Christian Heimes
 with contributions from Gregory P. Smith [Google] and Mark Shannon.

Python/clinic/sysmodule.c.h

@@ -2059,12 +2059,12 @@ sys.activate_stack_trampoline
     backend: str
     /
 
-Activate the perf profiler trampoline.
+Activate stack profiler trampoline *backend*.
 [clinic start generated code]*/
 
 static PyObject *
 sys_activate_stack_trampoline_impl(PyObject *module, const char *backend)
-/*[clinic end generated code: output=5783cdeb51874b43 input=b09020e3a17c78c5]*/
+/*[clinic end generated code: output=5783cdeb51874b43 input=a12df928758a82b4]*/
 {
 #ifdef PY_HAVE_PERF_TRAMPOLINE
     if (strcmp(backend, "perf") == 0) {
@@ -2095,12 +2095,12 @@ sys_activate_stack_trampoline_impl(PyObject *module, const char *backend)
 /*[clinic input]
 sys.deactivate_stack_trampoline
 
-Dectivate the perf profiler trampoline.
+Dectivate stack profiler trampoline backend.
 [clinic start generated code]*/
 
 static PyObject *
 sys_deactivate_stack_trampoline_impl(PyObject *module)
-/*[clinic end generated code: output=b50da25465df0ef1 input=491f4fc1ed615736]*/
+/*[clinic end generated code: output=b50da25465df0ef1 input=be0c9a8737fe7e8f]*/
 {
     if  (_PyPerfTrampoline_Init(0) < 0) {
         return NULL;
@@ -2111,12 +2111,12 @@ sys_deactivate_stack_trampoline_impl(PyObject *module)
 /*[clinic input]
 sys.is_stack_trampoline_active
 
-Returns *True* if the perf profiler trampoline is active.
+Return *True* if the stack profiler trampoline is active.
 [clinic start generated code]*/
 
 static PyObject *
 sys_is_stack_trampoline_active_impl(PyObject *module)
-/*[clinic end generated code: output=ab2746de0ad9d293 input=061fa5776ac9dd59]*/
+/*[clinic end generated code: output=ab2746de0ad9d293 input=d802fd4a1afa2de8]*/
 {
 #ifdef PY_HAVE_PERF_TRAMPOLINE
     if (_PyIsPerfTrampolineActive()) {